diff --git a/lib/sasacore/sasa.ml b/lib/sasacore/sasa.ml
index 2364638f9974a4a73737607934e7252880db54f3..29bcb9ffc29172523a8e036bd05e7b4b45cc5398 100644
--- a/lib/sasacore/sasa.ml
+++ b/lib/sasacore/sasa.ml
@@ -1,4 +1,4 @@
-(* Time-stamp: <modified the 26/03/2019 (at 17:20) by Erwan Jahier> *)
+(* Time-stamp: <modified the 27/03/2019 (at 17:28) by Erwan Jahier> *)
 
 open Algo
 open Sasacore
@@ -17,7 +17,7 @@ let (update_env_with_init : Env.t -> Process.t list -> Algo.neighbor list list -
 let (reply: Topology.t -> string -> string -> int) =
   fun g source target  ->
     let rec f i = function
-      | [] -> assert false
+      | [] -> (-1) (* may happen in directed graphs *) 
       | x::t -> if x=source then i else f (i+1) t
     in
     f 0 (g.succ target)